Monday evening, a strong low pressure center was located over eastern Lake Superior.
The circulation associated with this system was immense, extending from Hudson Bay in Canada, to the Gulf coast, and from the Rockies to the Appalachians. Polar air wrapping around the storm center swept across the Chicago area bringing plunging temperatures.
At 4 PM Sunday afternoon, the temperature at O'Hare airport stood at an April-level 54 degrees. By 4 PM Monday, it was 25 degrees colder, with light snow falling. Polar air will remain in place through New Year's Eve, providing a seasonable chill.
Clearing skies and slowly diminishing winds are expected to allow temps to dip to the low and mid 20s by the stroke of midnight.
The new year is forecast to dawn sunny and cold, but a return fetch of mild air is set to arrive the latter half of the week in advance of our next storm system.